Player Version#: 1.5.0.951
There was a problem reported back in Sep 2019 of a very small Plex Window being displayed when first opening Plex. Upon closing that small Window, the main Plex app would open. The issue was determined to be a scaling factor issue. That issue appears to be back once again.
More detail:
When I first launch the Plex app, this is what I see:

This issue was previously reported in this thread:
The solution was to make a change to Plex scaling as noted in this article:
I am again encountering this issue. In my case, the only Scaling Factor setting that will work is a value of 1.0. However, with this scaling factor set, everything in the Plex app appears unusually small in size. My screen is 15-inch high DPI, 4k laptop screen.
I’m just reporting this issue again because I’ve not noticed it in quite a while. I suspect that it may have been fixed at some point but that this may be a regression.